GAS-L: Re: Doubt

["Graeme Williams" <graeme@powerlink.co.nz>]

Hello Dinesh

Dual Fuel Engines

I'm surprised that you have appealed to the international gasification
community for assistance with your problem.  We are lead to believe that
India has a well established base of both commercial and academic expertise
in gasification and the answers to your questions is as close as the nearest
Professor or commercial installation.  Still, your questions have generated
a range of responses from our interest group and there would seem to be a
need to correct one or two points for the record.

Yes you can generate electrical power below 50% on dual fuel without
creating unstable combustion, and no it is not because the fuel charge is
too lean, but because there is insufficient air for the diesel and the
combustion is too rich.

In answer to your questions:

Q1: Before you can answer this question you have to know 50% of what?  Is it
the percentage based on maximum output capability on dual fuel at the
minimum diesel consumption i.e. 80% of the engines normal output on diesel,
whereby the gas replaces about 80% of the diesel normally consumed.  Or, the
amount of power available by restricting diesel flow to just achieve
ignition, which can be as low as 5% depending on the original size of the
injectors.

Either choice is determined by the circumstances of whether you are
measuring an existing engine or planning a project or just reviewing other
people's work, which has many pitfalls due to the lack of fixed
specifications of the equipment used.

The importance of knowing at what point of output to set the engine, to then
set up the gas/air control system is critical particularly if the system is
variable load rather than base load.  The reason for this is due to the
often forgotten fact that diesel fuel has a changing stortiometric ratio
that varies from about 20:1 on full load and over 100:1 at no load.  I also
should point out that the diesel fuel specifications are far from constant
in Asia and you would do well to factor this into any practical work you
initiate as part of your study.

Q2: Again you must qualify what is meant by peak loads of under 0.5, as it
refers to the maximum expected power demand, which for engine size relates
back to your choice of minimum diesel ignition or maximum power output.

The closest I can answer this question from practical experience is yes you
can operate a large engine (i.e. 100KVA) requiring two gasifiers at under
0.5 output on one gasifier.  This has more to do with maintaining gas
quality than anything else, and was appropriate for the size of gasifier we
manufactured at that time.

There is no reason why you cannot maintain dual fuel operation down to low
loadings, but it must not be below the gasifiers capability to sustain gas
quality at minimum output.  You also have to have a gas/air control system
that follows the load change to reduce the gas and increase the air, that is
if your system is unattended

Other factors:  Engines used for power generation on dual fuel must have a
fixed speed governor, and not a variable speed as used in most industrial or
truck engines.

Engine combustion stability can change if the gasifier makes intermittent
tar or pyrolysis oil even if on steady load.

Finally don't feel confused by all the calculations and formula in the
papers you read, more particularly so if the conclusions state further work
is required on the gas carburation.  The writer whoever it is, just doesn't
know how to control the gas/air system and any statements regarding dual
fuelling such as you have found are not logical based on many thousands of
dual fuelled variable load engine hours.  You can read about our engine work
on the Fluidyne archive  http://members.xoom.com/whitools/   for more
background information.

Trust this helps a little.

Regards
Doug Williams
Fluidyne Gasification.

> Date: Wed, 5 Apr 2000 11:28:00 -0700
> From: Dinesh Mahajan <dinesh@me.iitb.ernet.in>
> Subject: GAS-L: Doubt
>
> Dear Sir,
>
> I am a senior student at IIT Bombay, doing a study on dual-fuel operation
> of diesel engines (substitution of diesel by producer gas from biomas
> gasification). I had come across in literature that if the diesel engine
> is operating below 50% of its rated load, then dual-fuel operation is not
> possible, because the dual-fuel mixture becomes so lean that it can't
> support combustion.
>
> question 1> So if we use the option as a stand-alone power supplier would
> we have to shift to pure diesel mode in case the demand becomes less than
> 50% of the rated demand?
>
> question 2> Are their any case studies where the Gasifier-Diesel Engine
> energy route has been adopted to meet a load of peak load factor of below
> 0.5.
>
> I would be obliged if you could help me on this issue. I also wanted to
> know if I could contribute to your site by sharing my study on the
> gasifier based stand alone power systems.
>
>
> Thanks,
> Regards.
>
>
> DINESH MAHAJAN
> h-3,#227
> iit bombay
>
> 5779719


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